1. Field of the Invention
The present invention relates generally to an impedance matching means, and more particularly to an impedance matching means between an antenna and a transmission line.
2. Description of the Prior Art
In many RF situations, there is a relatively fixed load impedance—say a resonant antenna with a fixed impedance. A transmission line, such as a coaxial cable providing power energy to the antenna has its own characteristic impedance. In most cases when the energy reaches the end of the cable, we want as much as possible to transfer into our load—the antenna, in the case of a transmitter, or the input RF stage in the case of a receiver. For a transmitter this gives the highest power efficiency, while for a receiver this gives the best noise performance. To ensure this optimum energy transfer, we need to match the characteristic impedance of the cable to the impedance of the load. So for a 75 Ω antenna, we need to use 75 Ω cable. For a 50 Ω antenna we need to use 50 Ω cable, and so on. Impedance matching is critical factor in antenna assembly design. Because what happens if the transmission line and the antenna impedance are not matched is that some of the RF energy reaching the end of the transmission line cannot be transferred into the load, but is reflected back along the line towards the source. This can set up standing waves in the line and can also cause overheating in the transmitter output stage. In a receiver, the mismatch degrades the effective receiver gain and noise figure.
Hence, great attention is focused on the impedance matching by researchers in this field. Generally the cable impedance is more or less fixed, and the antenna impedance may be the same. So we need additional techniques to match the impedance of antenna with that of the cable.